1. Field of the Invention
The present invention relates to a printer of a thermal transfer type and more particularly to a printer for recording a multitone image.
2. Description of the Related Arts
A thermal transfer printing system can more readily deal with colors and can be made more compact than other printing systems like an ink-jet system or an electophotographic system, and because of its further advantages in image quality, cost, and maintenance, this system is widely applied to hard copy apparatus which record pictorial images.
Especially, a thermal transfer recording method which uses sublimating dye ink as thermosensible ink is suitable for recording a pictorial image. This method utilizes the characteristic of the sublimating dye that the amount of the dye to be transferred to the recording paper continuously changes according to the amount of a heating. Consequently, control of the recording density of a multitone image is possible by modulating the width of pulses to be supplied to a heating element of a thermal head. This density control method is superior to other density control methods such as a dither method or density pattern method with respect to forming a multitone image without a reduction in the resolution.
However, such a sublimation dye thermal transfer printer, which performs density control by the current pulse width modulation, has its recording density dependent on the ambient temperature, and it is therefore difficult to reproduce the density of the image correctly.
In full color recording, normally, the primary colors of yellow (Y), magenta (M), and cyan (C) are recorded one by one on one image plane, and the recording paper is rewound three times so as to superimpose recorded images of the three colors on each other. Unless the density of each color is correctly reproduced, the color of each pixel obtained by the mixture of the primary colors is different from a target color. Therefore, it is necessary to develop a temperature compensating technique to reproduce the density of the image by controlling the energy to be applied to the printer according to temperature so as to form an image of high quality.
In addition to the change in environmental temperature, the temperature rise (heat reserve) of the thermal head itself during the recording of the image is also a cause of a temperature change. Although heat generated in the heating element of the thermal head during the image recording is partly transmitted to an ink film, the generated heat is mostly transmitted to a head mount via a substrate of the heating element. As a result, during recording, there always exists nonconstant temperature distribution in a thermal head according to the input recording signals.
The temperature measurement for temperature compensation is carried out by a temperature measuring element such as a thermistor installed in the head mount spaced a certain distance from the heating element in order to prevent the measuring operation from giving a bad influence on the image recording. This is not enough to follow a temperature change in a portion disposed in the vicinity of the heating element such as the substrate of the heating element in response to recording signals. To cope with this problem, a temperature compensating method having a prompt response to the temperature change has been proposed.
According to U.S. Pat. No. 5,066,961, the temperature of the substrate of the heating element is evaluated based on the measured temperature of the head mount of the thermal head and energy applied to the heating elements from the first line until a preceding line so as to calculate a compensation coefficient from the temperature of the substrate of the heating element. Then, the compensation coefficient is multiplied by the pulse width data. In this manner, a temperature compensation is carried out.
In the above-described apparatus, temperature compensation is performed by multiplying the compensation coefficient by the pulse width data. This method has, however, a problem that during the high speed recording, a temperature compensation cannot be accomplished accurately. Therefore, the density of the multitone image cannot be recorded favorably when a temperature has changed.